In gas turbine engines, for example, aircraft engines, air is drawn into the front of the engine, compressed by a shaft-mounted rotary compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on a shaft. The flow of gas turns the turbine, which turns the shaft and drives the compressor. The hot exhaust gases flow from the back of the engine, providing thrust that propels the aircraft forward.
During operation of gas turbine engines, the temperatures of combustion gases may exceed 3,000 degrees F., considerably higher than the melting temperatures of the metal parts of the engine, which are in contact with these gases. The metal parts that are particularly subject to temperature extremes and degradation by the oxidizing and corrosive environment, and thus require particular attention with respect to cooling, are the hot section components exposed to the combustion gases, such as blades and vanes used to direct the flow of the hot gases, as well as other components such as shrouds and combustors.
The hotter the exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the exhaust gas temperature. However, the maximum temperature of the exhaust gases is normally limited by the materials used to fabricate the hot section components of the turbine. In current engines, hot section components such as the turbine vanes and blades are made of cobalt-based and nickel-based superalloys, and can operate in temperature ranges of 2000°-2300° F. and higher.
The metal temperatures can be maintained below melting levels with current cooling techniques by using environmental coatings alone or in combination with thermal barrier coatings (TBCs). TBCs includes a ceramic thermal barrier coating that is applied to the external surface of metal parts within engines to impede the transfer of heat from hot combustion gases to the metal parts, thus insulating the component from the hot gases. This permits the exhaust gas to be hotter than would otherwise be possible with the particular material and fabrication process of the component.
TBCs are well-known ceramic coatings, for example, yttrium stabilized zirconia. Ceramic TBCs usually do not adhere optimally directly to the superalloys used in the substrates. Therefore, an environmental metallic coating called a bond coat is placed between the substrate and the TBC to improve adhesion of the TBC to the underlying component. The bond coat temperature is critical to the life of the TBC and has been limited to about 2100° F. Once the bond coat exceeds this temperature, the coating system can quickly deteriorate, resulting in spallation of the TBC from the bond coat.
These bond coats are typically applied by several suitable techniques. The coating may be produced by manufacturing a powder of the appropriate composition, then using a thermal spray technique, such as, for example, low pressure plasma spray (LPPS), high velocity oxy-fuel (HVOF) and D-Gun to apply the powder to the airfoil to form a thin coating. These processes tend to be line of sight processes and are effective for smooth regular surfaces, but are not as effective for complex shapes such as air foils.
Coatings also may be applied by diffusion processes such as vapor phase aluminiding (VPA), co-deposition techniques (CODEP), and similar processes. In these processes, the part to be coated is heated to an elevated temperature in a retort having an atmosphere rich in a certain element or elements, often aluminum, in which a partial pressure of an aluminum or other desired vapor is developed in the retort. These elements diffuse into the surface of the part to form a diffusion coating.
A coating can be applied by chemical vapor deposition (CVD) in which a vapor of the desired elements required to produce the coating are formed in a chamber external to the part to be coated. This vapor is then introduced into a second chamber containing the part, wherein the vapor is deposited on the part.
A coating also may be applied by an electroplating technique whereby the part to be coated is immersed in a bath containing metallic ions, such as Pt, Rh, Pd, Ni, which are then transferred to the surface of the part by the passage of an electric current. Combinations of electroplating and the previously discussed processes may be used. For example, Pt, Ni, or any combination thereof may first be applied to a substrate by electrodeposition. Aluminum may then be applied by a technique such as VPA to produce a modified PtAl coating.
Another bond coat utilizes a MCrAlY(X) where M is an element selected from Fe, Co and Ni and combinations thereof and (X) is an optional element selected from gamma prime formers, solid solution strengtheners, grain boundary strengtheners and combinations thereof. The MCrAlY(X) is applied by, for example, physical vapor deposition (PVD) processes such as electron beam (EB), ion-plasma thermal spray, or sputtering.
Whereas a MCrAlY(X) provides good resistance to high temperature hot corrosion, it does not provide good resistance to high temperature oxidation. In comparison, diffusion coatings applied by CVD and PVD have increased resistance to high temperature oxidation. In particular, diffusing Al into the substrate has proven effective against high temperature oxidation. The CVD bond coat forms an aluminum oxide scale during exposure to oxygen containing atmospheres at elevated temperatures. A coating of intermetallic β phase NiAl has been found to be most effective at increasing both hot corrosion resistance and high temperature oxidation. However, a β phase NiAl is, by nature, a brittle substance with little ductility. A TBC applied over the bond coat allows the article to achieve even higher operating temperatures. Technologies that allow longer life for such coatings, or allow higher operating temperatures, are of significant benefit. In U.S. Pat. No. 6,153,313, assigned to the assignee of the present invention and incorporated by reference herein, Rigney et al. teaches that a β phase NiAl coating (about 30 to 60 atomic percent aluminum) containing alloying additions of rare earth and other elements that increase the creep strength of a bond coat result in both improved spallation and oxidation resistance. However, Rigney does not teach the methodology by which this bond coat might be successfully applied to a complexly-shaped article of commercial interest, such as a turbine airfoil.
Considerable difficulty is encountered in processing an intermetallic coating such as NiAl by conventional means, such as thermal spray. Of course, diffusion methods produce a gradient of compositions and the not the desired discrete B-phase intermetallic. Known powder electroplating techniques allow such a coating to be applied with little to no deformation of the coating.
Coating an article by depositing a coating layer or series of coating layers by electroplating and then depositing a second coating layer or series of coating layers on the electroplated coating layer by physical vapor deposition is known in the art. For example, U.S. Pat. No. 5,879,532 teaches electroplating a brass article, pulse blow drying the article after electroplating, and then adding additional coats using a vapor deposition process.
Additionally, U.S. Pat. Nos. 4,789,441; U.S. Pat. No. 4,810,334; and U.S. Pat. No. 5,833,829 all teach methods of entrapment plating whereby a metal or metals are electroplated on the surface of an article. Simultaneously, a powder or powders are present in the plating bath, and during plating these powder particles become entrapped in the plate. A coating having some fraction of powder particles entrapped in the metallic matrix is obtained. U.S. Pat. No. 4,789,441 patent teaches subsequent heat treatment of the coating to obtain interdiffusion between the constituents of the matrix and the particles.
However, a level of Al in the powder sufficient to give the amount required to achieve the predominantly β phase NiAl coating of Rigney with its increased resistance to high temperature oxidation would be unstable in a standard plating solution. While U.S. Pat. No. 5,833,829 patent teaches an optional subsequent aluminizing process using a pack or vapor phase aluminizing process, it does not recognize or try to achieve the advantageous qualities of the β phase of NiAl as taught by Rigney.
What is needed are improved methods to apply a bond coat to obtain the β phase of NiAl taught by Rigney. The present invention fulfills this need, and further provides related advantages by conceiving a method to produce a bond coat to achieve the benefits taught by Rigney using an electroplating technique.